Variable speed drive with active converter

ABSTRACT

A system or method for a VSD with an active converter including a controller, an inductor, an active converter, a DC link, and an inverter. The active converter is controlled to receive an input AC voltage and output a boosted DC voltage to a DC link, up to 850 VDC, the active converter using only low voltage semiconductor switches to provide the 850 VDC DC link voltage. The controller is configured to operate with a reactive input current magnitude equal to zero at a predetermined system load, and at system loads less than the predetermined system load, to introduce a reactive input current that results in a converter voltage having a magnitude less than the input voltage, wherein the vector sum of the input voltage and an inductor voltage is equal to the converter voltage.

This application claims priority from and the benefit of U.S.Provisional Patent Application No. 62/158,749, entitled VARIABLE SPEEDDRIVE WITH ACTIVE CONVERTER, filed May 8, 2015, which is herebyincorporated by reference.

BACKGROUND

The application generally relates to a variable speed drive. Theapplication relates more specifically to a variable speed drive with anactive converter, and a control method for improved operatingefficiency.

Variable speed drives (VSD) are commonly used for controlling theoperating speed of synchronous and asynchronous motors. A VSD includes aconverter for converting an AC line input voltage to a DC voltage, a DClink bus with a DC bus and capacitor storage and an inverter to providevariable AC output power to a motor load. The converter may be a passiveor active converter. If an active converter is provided, certainbenefits can be attained, such as controlling the operating power factorof the motor and associated equipment, and reducing harmonic noise.

The active converter typically includes semiconductor switches, such asIGBTs which switch currents to achieve low harmonic input current and DCbus voltage. The voltage rating of the DC bus is a function of thesource input voltage to the VSD and the output voltage that the VSDprovides to the motor. In commercial and industrial equipment, higherinput and output voltage are required, and IGBT modules rated for 1700Volts, or high voltage IGBT, are normally required to meet the desiredoutput voltage to drive the motor. Other switches may also be used,including but not limited to MOSFETs, SiC MOSFETs, and GaN transistors.IGBTs rated for 1700 Volts, generate increased switching losses,resulting in reduced efficiency characteristics of the VSDs. Bycontrast, 1200V IGBT modules, or low voltage IGBT, are characterized bylower switching losses and increased efficiency, but are limited to alower DC bus voltage.

Currently three different solutions are used to control VSDs. Someapplications employ three levels of switches in the VSD converter, whichrequires twice the number of IGBTs when rated at lower voltages. Lowvoltage IGBTs may be used in that case, although this method increasesthe complexity and cost of the VSD. In another solution, a passive frontend may be employed, however harmonic currents will be reflected backinto the power system and require additional filters at the voltagesource to meet the harmonic standards. Lastly, a third approach employshigh voltage IGBT modules, resulting in higher losses and additionalcost associated with the VSD.

The disclosure provides a method and system to reduce the DC bus voltageof a VSD to accommodate low voltage IGBTs to be used in the VSD whilestill achieving the DC link voltage sufficient to provide an increasedAC voltage output from the inverter.

Intended advantages of the disclosed systems and/or methods satisfy oneor more of these needs or provide other advantageous features. Otherfeatures and advantages will be made apparent from the presentspecification. The teachings disclosed extend to those embodiments thatfall within the scope of the claims, regardless of whether theyaccomplish one or more of the aforementioned needs.

SUMMARY

One embodiment relates to a VSD with an active converter including acontroller, an inductor, a power stage, a DC link, and an inverter. Theactive converter is controlled to receive an input AC voltage and outputa boosted DC voltage. The input voltage may vary from 240V to 635V rmsto regulate the DC link up to 850 VDC. The inverter converts thisvoltage to AC from 0 to 575 Volt. The controller is normally configuredto operate with a reactive input current magnitude equal to zero.

Certain advantages of the embodiments described herein include asolution that may be implemented through a strategic softwaremodification to control the active converter to add a reactive currentcomponent to the input current of the VSD.

Another advantage of the disclosure is a reduced DC bus operatingvoltage of the voltage source converters, allowing the use of IGBTmodules rated for a lower voltage, e.g., 1200 V, on the input and outputends of the VSD, and lower losses as compared to inefficient highvoltage IGBTs.

Still another advantage is increased operating efficiency of the VSD.

Alternative exemplary embodiments relate to other features andcombinations of features as may be generally recited in the claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates schematically a general system configuration of thepresent invention.

FIG. 2 illustrates schematically an embodiment of variable speed driveof the present invention.

FIG. 3 illustrates schematically a refrigeration system that can be usedwith the present invention.

FIG. 4 illustrates an exemplary embodiment of an active converterarrangement.

FIG. 5 illustrates an exemplary PWM switching network for implementingan active converter with reduced voltage semiconductor switches.

FIG. 6 is a vector representation of an exemplary VSD operation at fullload.

FIG. 7 is a vector representation of VSD an exemplary operation at lightload.

FIG. 8 is a vector representation of VSD operation at light load withreactive current inserted into the PWM modulation algorithm.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIGS. 1 and 2 illustrate generally the system configuration of thepresent invention. An AC power source 102 supplies AC power to the VSD104, which in turn, supplies AC power to a motor 106. The motor 106 ispreferably used to drive a corresponding compressor of a refrigerationor chiller system. The AC power source 102 provides three-phase, fixedvoltage, and fixed frequency AC power to the VSD 104 from an AC powergrid or distribution system that is present at a site. The AC power gridcan be supplied directly from an electric utility or can be suppliedfrom one or more transforming substations between the electric utilityand the AC power grid. The AC power source 102 can preferably supply athree phase AC voltage or line voltage of 200 V, 230 V, 380 V, 460 V, or600 V, at a line frequency of 50 Hz or 60 Hz to the VSD 104 depending onthe corresponding AC power grid. It is to be understood that the ACpower source 102 can provide any suitable fixed line voltage or fixedline frequency to the VSD 104 depending on the configuration of the ACpower grid. In addition, a particular site can have multiple AC powergrids that can satisfy different line voltage and line frequencyrequirements. For example, a site may have 230 VAC power grid to handlecertain applications and a 460 VAC power grid to handle otherapplications.

Referring next to FIG. 2, the VSD 104 receives AC power having aparticular fixed line voltage and fixed line frequency from the AC powersource 102 and provides AC power to the motor 106 at a desired voltageand desired frequency, both of which can be varied to satisfy particularrequirements. Preferably, the VSD 104 can provide AC power to the motor106 having higher voltages and frequencies or lower voltages andfrequencies than the fixed voltage and fixed frequency received from theAC power source 102. FIG. 2 illustrates schematically some of thecomponents in one embodiment of the VSD 104. The VSD 104 can have threestages: a converter 202, a DC link 204 and an inverter 206. Theconverter 202 converts the fixed line frequency, fixed line voltage ACpower from the AC power source 102 into DC power. The DC link 204filters the DC power from the converter 202 and provides energy storagecomponents such as capacitors 208 and/or inductors (not shown). Theinverter 206 converts the DC power from the DC link 204 into variablefrequency, variable voltage AC power for the motor 106.

The motor 106 may be an induction motor that is capable of being drivenat variable speeds. The induction motor can have any suitable polearrangement including two poles, four poles or six poles. The inductionmotor is used to drive a load, preferably a compressor of arefrigeration or chiller system as shown in FIG. 3. FIG. 3 illustratesgenerally the system of the present invention connected to arefrigeration system.

As shown in FIG. 3, the HVAC, refrigeration or liquid chiller system 300includes a compressor 302, a condenser 304, an evaporator 306, and acontrol panel 308. The control panel 308 can include a variety ofdifferent components such as an analog to digital (A/D) converter, amicroprocessor, a non-volatile memory, and an interface board, tocontrol operation of the refrigeration system 300. The control panel 308can also be used to control the operation of the VSD 104 and the motor106.

Compressor 302 compresses a refrigerant vapor and delivers the vapor tothe condenser 304 through a discharge line. The compressor 302 ispreferably a centrifugal compressor, but can be any suitable type ofcompressor, e.g., screw compressor, reciprocating compressor, etc. Therefrigerant vapor delivered by the compressor 302 to the condenser 304enters into a heat exchange relationship with a fluid, e.g., air orwater, and undergoes a phase change to a refrigerant liquid as a resultof the heat exchange relationship with the fluid. The condensed liquidrefrigerant from condenser 304 flows through an expansion device (notshown) to an evaporator 306.

The evaporator 306 can include connections for a supply line and areturn line of a cooling load. A secondary liquid, e.g., water,ethylene, calcium chloride brine or sodium chloride brine, travels intothe evaporator 306 via return line and exits the evaporator 306 viasupply line. The liquid refrigerant in the evaporator 306 enters into aheat exchange relationship with the secondary liquid to lower thetemperature of the secondary liquid. The refrigerant liquid in theevaporator 306 undergoes a phase change to a refrigerant vapor as aresult of the heat exchange relationship with the secondary liquid. Thevapor refrigerant in the evaporator 306 exits the evaporator 306 andreturns to the compressor 302 by a suction line to complete the cycle.It is to be understood that any suitable configuration of condenser 304and evaporator 306 can be used in the system 300, provided that theappropriate phase change of the refrigerant in the condenser 304 andevaporator 306 is obtained.

The HVAC, refrigeration or liquid chiller system 300 can include manyother features that are not shown in FIG. 3. These features have beenpurposely omitted to simplify the drawing for ease of illustration.Furthermore, while FIG. 3 illustrates the HVAC, refrigeration or liquidchiller system 300 as having one compressor connected in a singlerefrigerant circuit, it is to be understood that the system 300 can havemultiple compressors, powered by a single VSD or multiple VSDs,connected into each of one or more refrigerant circuits.

Preferably, a control panel 308, microprocessor or controller canprovide control signals to the VSD 104 to control the operation of theVSD 104 (and possibly motor 106) to provide the optimal operationalsetting for the VSD 104 and motor 106 depending on the particular sensorreadings received by the control panel 308. For example, in therefrigeration system 300 of FIG. 3, the control panel 308 can adjust theoutput voltage and frequency of the VSD 104 to correspond to changingconditions in the refrigeration system, i.e., the control panel 308 canincrease or decrease the output voltage and frequency of the VSD 104 inresponse to increasing or decreasing load conditions on the compressor302 in order to obtain a desired operating speed of the motor 106 and adesired load output of the compressor 302.

Referring back to FIG. 2, the converter 202 may be a pulse widthmodulated (PWM) boost converter or rectifier having insulated gatebipolar transistors (IGBTs) to provide a boosted DC voltage to the DClink 204 to obtain a maximum fundamental RMS output voltage from the VSD104 greater than the nominal RMS fundamental input voltage of the VSD104. In a preferred embodiment of the present invention, the VSD 104 canprovide a maximum output voltage that is greater than the fixed nominalfundamental RMS input voltage provided to the VSD 104 and a maximumfundamental RMS output frequency that is greater than the fixed inputfrequency provided to the VSD 104. Furthermore, it is to be understoodthat the VSD 104 can incorporate different components from those shownin FIG. 2 so long as the VSD 104 can provide the motor 106 withappropriate output voltages and frequencies.

Referring next to FIG. 4, one embodiment of the active converter orrectifier module 202 is shown. One of the power switches in each pair ofpower switches is an IGBT 450 connected to an inverse or anti-paralleldiode 452. The inverse or anti-parallel diode 452 is used to conductcurrent after the other power switch, IGBT 450, is turned off when theVSD 104 is operated in a pulse width modulation mode. The IGBTs 450 andinverse diodes 452 are connected between the output of the circuitprotective devices and three-phase line inductor 416 and the negativerail of the DC bus 412. However, in another embodiment of the presentinvention, a multiplicity of IGBTs 450 and inverse diodes 452 can beconnected between the output of the circuit protective devices andthree-phase line inductor 416 and the positive rail of the DC bus 412 a,as shown in FIG. 4. Circuit protective devices 416 may includeinductors, circuit breakers, fuses and other apparatus for protectingthe VSD circuit components connected to the load side of the devices416.

Referring next to FIG. 5, a pulse width modulation technique isimplemented in the converter 202 by controller 308 to provide athree-phase boost rectifier. Closed loop control of the three-phase PWMboost rectifier may be implemented in a digital signal processor (DSP)located in the controller. For one exemplary embodiment of a PWM controlmethod, see, e.g., “Modeling and Control of Three-Phase PWM Converters,”Silva Hiti, Ph. D. Dissertation, Virginia Polytechnic Institute andState University, Blacksburg, Va. 1995.

The power stage of a three-phase boost rectifier/converter 202 includesa three-phase switching network. This switching network requires six PWMgating signals 12 generated by a PWM modulator 10. PWM modulator 10generates the gate signals 12 based on the inputs (d_(q) and d_(d))provided by the boost rectifier's control loops. Boost rectifieroperation may be enabled after the semiconductor pre-charge devices aregated continuously on or a supply contactor (not shown) is closed forconverters without auxiliary IGBTs. In one embodiment, PWM modulator 10receives the signals d_(α) _(_) _(sat), and d_(β) _(_) _(sat□□□) Signalsd_(α) _(_) _(sat), and d_(β) _(_) _(sat□□) are obtained from acoordinate transformation 14 using transformations from a Cartesiancoordinate system to a polar coordinate system. Gating signals 12 may berepresented by the variables g_(ap), g_(an), g_(bp), g_(bn), g_(cp), andg_(cn), where g_(xy) represents the respective gating signal, xindicates the phase of the rectifier, and y indicates whether it is agating signal for an IGBT connected to the positive dc rail (p) or thenegative dc rail (n).

The PWM modulator method may be selected based on the VSD size and fullload ampere (FLA) setting. If FLA is lower than a predeterminedthreshold the PWM modulator may be continuous space vector modulation;if FLA is greater than or equal to the predetermined threshold, adiscontinuous space vector modulation may be applied. The PWM modulatoroperating in overmodulation mode may also be used to achieve stable DCbus voltage at higher input power line voltage which extends thestability range of the VSD.

The PWM modulator utilizes space vector modulation (SVM). In SVM, d_(α)_(_) _(sat) and d_(β) _(_) _(sat) are defined as a real and an imaginarypart of a vector d in a complex plane (d_(α)+j·d_(β)). Gating signalsg_(ap), g_(an), g_(bp), g_(bn), g_(cp), and g_(cn) are then generatedbased on the magnitude and phase of d. SVM operates on an up-downcounter whose frequency is the boost rectifier's switching frequency.The period of the counter is the switching period, or switching cycle,T_(sw), of the rectifier. One half of the switching period T_(sw) is thesampling period, T_(smp). In each sampling period, the gating signalsg_(ap), g_(an), g_(bp), g_(bn), g_(cp), and g_(cn) are determined forthe sampling period that follows.

Control system 308 also includes a power system voltage phase trackingalgorithm 18, which senses input voltage ν_(A) and ν_(B) at sensingmodule 20. Input current is sensed at sensing module 24, and voltage onthe DC link is sensed at sensing module 26. Sensed data filters 22 areapplied to input voltages and currents and transformed into the desiredformat, e.g., from Cartesian coordinates, or abc, into polarcoordinates, or stationary dq coordinates, also referred to as d-channeland q-channel voltages and currents.

In order to achieve unity power factor, conventional PWM controllersmatch the phase of the active converter line current to the phase of thesource voltage. In order to accomplish unity power factor, the referencecurrent i_(q) _(_) _(ref) is set to zero in the current sensing loop ofthe power system reactive and active current components controls module16. Active converter 202 is configured to provide a boosted DC voltageon the DC link, to 850 V. At 635V power supply input voltage at fullload operation, the converter is able to operate within the rated limitfor semiconductor switches rated at 1200 V and still maintain stable DClink voltage of, for example, 850 VDC. This is normally due to thevoltage drop across the power supply impedance. However, at reducedloads, in order to maintain the inverter output at acceptable voltagelevels, the DC bus voltage must be increased to 870 VDC, which under theconventional operating parameters would require a higher ratedvoltage—i.e., 1700 V—for the semiconductor switches.

In order to maintain the same output voltage and regulate the DC link ata maximum of 850 VDC, the reference current i_(q) _(_) _(ref) may be setto a non-zero magnitude, or value, adding a reactive component to thecurrent through the three phase switching network 16.

FIGS. 6 and 7 illustrate vector representations for VSD operation at635V input rms line voltage, full load (FIG. 6) and at light or reducedloads (FIG. 7), respectively. In FIG. 6, with the VSD operating at ornear full load, a DC bus voltage of 850 VDC is attainable usingovermodulation techniques, while maintaining a stable operation of VSD104. FIG. 6 shows exemplary full load parameters, with the VSD operatingat a full load current of approximately 1410 amperes. The AC sourcevoltage (V_(inp)) 102 applied to inductor L_(c) is 498.4V phase voltagepeak. The voltage drop (V_(ind)) across inductor L_(c) is 22.3V phasevoltage peak, with a phase angle that is orthogonal to the input voltagevector (V_(inp)), and the sum of the vectors V_(inp) and V_(ind) yieldsthe converter voltage V_(conv), of 498.9 phase voltage peak. Theconverter voltage of 498.9 volts is thus achievable in the example shownin FIG. 6, using PWM overmodulation technique. Thus at full loadoperation the VSD maintains stable operation.

Referring next to FIG. 7, in this example VSD 104 is operating at alight load, e.g., 89.73 amperes of input current. To control the DC linkat 850 VDC, V_(inp) is 514V peak, and V_(ind) is 1.42V peak. Thus,V_(conv) must be 514V peak, a magnitude that is not achievable with orwithout overmodulation. For V_(conv) to be controlled to 514 V, the DCbus voltage must be increased to 870 VDC, which would normally requiresemiconductor switches rated at 1700V to be used to achieve acceptablereliability. Since 1700V semiconductor switches are not desirable forthe VSD application, an alternative method is disclosed for achievingthe desired operating parameters.

Referring to FIG. 8, to achieve stable converter operation at 850V DCbus voltage, the reactive current may be introduced which lowers therequired converter voltage (V_(conv)) to the value achievable with 850VDC bus. The voltage vector V_(ind) across the inductor and V_(imp) yieldthe required converter voltage vector (V_(conv)). With reactive currentadded to the system, the vector sum of the input inductor and input linevoltages yields lower V_(conv) voltage (See FIG. 8). In the exampleshown in FIG. 8, adding a reactive current component, Ir, of 92 amperes,V_(ind) is phase shifted by an angle that is 90° with respect to thecurrent vector (I_(total)), which results in V_(conv) of 512.6Vachievable with 850V DC bus. The reactive current component of I_(inp)is produced by inserting a predetermined magnitude of the referencecurrent for the quadrature current (i_(q) _(_) _(ref)) in thecontroller, that produces the desired phase shift Θ for V_(ind).

The control system 308 monitors the DC ripple on the DC link voltage asan indication of the stability of the variable speed drive 104. When theload on the variable speed drive is high, the ripple on the DC linkvoltage is relatively small and the variable speed drive system isstable as indicated in the description of FIGS. 6 through 8. When theload on the variable speed drive is high and decreasing, the ripple onthe DC link voltage will eventually exceed a predetermined thresholdripple. The power system reactive and active current components control16 begins to inject reactive current to stabilize the DC link voltage.Further load decreases result in the ripple on the DC link voltageeventually exceeding the predetermined threshold ripple, necessitating afurther increase of reactive current being injected by the power systemreactive and active current components control 16 to stabilize the DClink voltage and thus stability of the variable speed drive 104.

Conversely, when the load on the variable speed drive 104 is low andincreasing, when the DC ripple at a level of injected reactive currentdecreases below a second predetermined threshold ripple, the secondpredetermined threshold ripple being less than the predeterminedthreshold ripple, the power system reactive and current componentscontrol 16 reduces the magnitude of injected current. This process mayoccur in a sequence of steps until an injected reactive current is nolonger required to maintain stability of the DC link voltage and thusstability of the variable speed drive. See the description of FIGS. 6through 8 relating to the operation of variable speed drive 104.

While an exemplary version of PWM is disclosed above, the disclosedmethods and systems are not limited to a particular PWM method. OtherPWM methods are disclosed in “A Comparative Study of Control Techniquesfor PWM Rectifiers in AC Adjustable Speed Drives”, M. Malinowski et al.,IEEE Transactions on Power Electronics, Vol. 18, No. 6 (November 2003).

It should be understood that the application is not limited to thedetails or methodology set forth in the description or illustrated inthe figures. It should also be understood that the phraseology andterminology employed herein is for the purpose of description only andshould not be regarded as limiting.

While the exemplary embodiments illustrated in the figures and describedherein are presently preferred, it should be understood that theseembodiments are offered by way of example only. Accordingly, the presentapplication is not limited to a particular embodiment, but extends tovarious modifications that nevertheless fall within the scope of theappended claims. The order or sequence of any processes or method stepsmay be varied or re-sequenced according to alternative embodiments.

The present application contemplates methods, systems and programproducts on any machine-readable media for accomplishing its operations.The embodiments of the present application may be implemented using anexisting computer processors, or by a special purpose computer processorfor an appropriate system, incorporated for this or another purpose orby a hardwired system.

It is important to note that the construction and arrangement of thevariable speed drive as shown in the various exemplary embodiments isillustrative only. Although only a few embodiments have been describedin detail in this disclosure, those who review this disclosure willreadily appreciate that many modifications are possible (e.g.,variations in sizes, dimensions, structures, shapes and proportions ofthe various elements, values of parameters, mounting arrangements, useof materials, colors, orientations, etc.) without materially departingfrom the novel teachings and advantages of the subject matter recited inthe claims. For example, elements shown as integrally formed may beconstructed of multiple parts or elements, the position of elements maybe reversed or otherwise varied, and the nature or number of discreteelements or positions may be altered or varied. Accordingly, all suchmodifications are intended to be included within the scope of thepresent application. The order or sequence of any process or methodsteps may be varied or re-sequenced according to alternativeembodiments. In the claims, any means-plus-function clause is intendedto cover the structures described herein as performing the recitedfunction and not only structural equivalents but also equivalentstructures. Other substitutions, modifications, changes and omissionsmay be made in the design, operating conditions and arrangement of theexemplary embodiments without departing from the scope of the presentapplication.

It should noted that although the figures herein may show a specificorder of method steps, it is understood that the order of these stepsmay differ from what is depicted. Also two or more steps may beperformed concurrently or with partial concurrence. Such variation willdepend on the software and hardware systems chosen and on designerchoice. It is understood that all such variations are within the scopeof the application. Likewise, software implementations could beaccomplished with standard programming techniques with rule based logicand other logic to accomplish the various connection steps, processingsteps, comparison steps and decision steps.

What is claimed is:
 1. A variable speed drive, comprising: an inductorconfigured to be electrically coupled to an alternating current powersource; an active converter electrically coupled to the inductor,wherein the active converter is configured to convert input alternatingcurrent electrical power received from the inductor into first directcurrent electrical power that comprises a direct current voltage; adirect current (DC) link electrically coupled to the active converter,wherein the DC link is configured to: store electrical energy based atleast in part on the first direct current electrical power received fromthe active converter; and output second direct current electrical powerby filtering the direct current voltage using the electrical energystored by the DC link; an inverter electrically coupled to the DC link,wherein the inverter is configured to convert the second direct currentelectrical power received from the DC link into output alternatingcurrent electrical power to be supplied to an electrical load; and acontrol system communicatively coupled to the active converter, whereinthe control system is configured to: determine a DC link voltage presenton the DC link; and output control signals that control operation of theactive converter to: maintain the DC link voltage below a DC linkvoltage threshold; introduce a first non-zero reactive current when thecontrol system determines that magnitude of a ripple on the DC linkvoltage exceeds a first ripple threshold; and introduce a secondnon-zero reactive current when control system determines that magnitudeof the ripple on the DC link voltage exceeds a second ripple threshold,wherein magnitude of the second non-zero reactive current is greaterthan magnitude of the first non-zero reactive current when the secondripple threshold is greater than the first ripple threshold.
 2. Avariable speed drive as recited in claim 1, comprising a sensing moduleelectrically coupled to the DC link and communicatively coupled to thecontrol system, wherein: the sensing module is configured to outputsensed data indicative of the DC link voltage; and the control system isconfigured to determine the DC link voltage based at least in part onthe sensed data.
 3. The variable speed drive of claim 1, wherein the DClink voltage threshold is 850 volts DC.
 4. The variable speed drive ofclaim 1, wherein the active converter is implemented as a pulse widthmodulated rectifier.
 5. The variable speed drive of claim 4, wherein theactive converter is configured to operate in an overmodulation mode. 6.The variable speed drive of claim 1, wherein the output alternatingcurrent electrical power comprises an alternating current voltage. 7.The variable speed drive of claim 1, wherein the active convertercomprises a plurality of solid state switching devices.
 8. A variablespeed drive system, comprising: a closed refrigerant loop, wherein theclosed refrigerant loop comprises a compressor, a condenser, and anevaporator; a motor mechanically coupled to the compressor; a variablespeed drive electrically coupled to the motor, wherein the variablespeed drive comprises: an inductor configured to be electrically coupledto an alternating current power source; an active converter electricallycoupled to the inductor, wherein the active converter is configured toconvert input alternating current electrical power received from theinductor into first direct current electrical power that comprises adirect current voltage; a direct current (DC) link electrically coupledto the active converter, wherein the DC link is configured to: storeelectrical energy based at least in part on the first direct currentelectrical power received from the active converter; and output seconddirect current electrical power by filtering the direct current voltageusing the electrical energy stored by the DC link; an inverterelectrically coupled to the DC link, wherein the inverter is configuredto convert the second direct current electrical power received from theDC link into output alternating current electrical power to be suppliedto an electrical load; and a controller communicatively coupled to theactive converter, wherein the controller is configured to: determine aDC link voltage present on the DC link; and output control signals thatcontrol operation of the active converter to: maintain the DC linkvoltage below a DC link voltage threshold; introduce a first non-zeroreactive current when the controller determines that magnitude of aripple on the DC link voltage exceeds a first ripple threshold; andintroduce a second non-zero reactive current when controller determinesthat magnitude of the ripple on the DC link voltage exceeds a secondripple threshold, wherein magnitude of the second non-zero reactivecurrent is greater than magnitude of the first non-zero reactive currentwhen the second ripple threshold is greater than the first ripplethreshold.
 9. A variable speed drive system of claim 8, comprising asensing module electrically coupled to the DC link and communicativelycoupled to the controller, wherein: the sensing module is configured tooutput sensed data indicative of the DC link voltage; and the controlleris configured to determine the DC link voltage based at least in part onthe sensed data.
 10. The variable speed drive system of claim 8, whereinthe DC link voltage threshold is 850 volts DC.
 11. The variable speeddrive system of claim 8, wherein the active converter is implemented asa pulse width modulated rectifier.
 12. The variable speed drive systemof claim 11, wherein the active converter is configured to operate in anovermodulation mode.
 13. The variable speed drive system of claim 8,wherein: the output alternating current electrical power comprise analternating current voltage; and the active converter comprises aplurality of solid state switching devices.
 14. A method for operating avariable speed drive, comprising: instructing, using at least oneprocessor, an active converter to convert input alternating currentelectrical power received from an inductor electrically coupled betweenthe active converter and an alternating current power source into firstdirect current electrical power supplied to a DC link; instructing,using the at least one processor, an inverter electrically coupled tothe DC link to convert second direct current electrical power receivedfrom the DC link into output alternating electrical power supplied to anelectrical load; and determining, using the at least one processor, a DClink voltage based at least in part on sensed data received from asensing module electrically coupled to the DC link; determining, usingthe at least one processor, whether magnitude of a ripple on the DC linkvoltage exceeds a first ripple threshold; and determining, using the atleast one processor, whether magnitude of a ripple on the DC linkvoltage exceeds a second ripple threshold; wherein instructing theactive converter to convert the input alternating current electricalpower into the first direct current electrical power comprises:instructing the active converter to generate the first direct currentelectrical power with a direct current voltage that facilitatesmaintaining the DC link voltage below a DC link voltage threshold;instructing the active convert to introduce a first non-zero reactivecurrent when the magnitude of the ripple on the DC link voltage exceedsa first ripple threshold; and instructing the active convert tointroduce a second non-zero reactive current reactive current whenmagnitude of the ripple on the DC link voltage exceeds the second ripplethreshold.
 15. The method of claim 14, wherein magnitude the secondnon-zero reactive current is greater than magnitude of the firstnon-zero reactive current when the second ripple threshold is greaterthan the first ripple threshold.
 16. The method of claim 14, wherein theDC link voltage threshold is 850 volts DC.